1. Field
At least one embodiment generally relates to electronic systems, and more specifically to systems using electromagnetic transponders, that is, transceivers capable of being interrogated contactless and wireless by a read and/or write terminal.
2. Discussion of the Related Art
There are many communication systems based on a modulation of an electromagnetic field generated by a terminal. They range from a simple electronic tag used as a theft-prevention device to more complex systems where a transponder intended to communicate with the terminal when present in the field thereof, is equipped with calculation functions (electronic purse, for example) or data processing functions.
Electromagnetic transponder systems are based on the use of oscillating circuits comprising a winding forming an antenna, on the transponder side and on the terminal side. Such circuits are intended to be coupled by near magnetic field when the transponder enters the field of the terminal. The oscillating circuits of the terminal and of the transponder are generally tuned to a same frequency corresponding to the excitation frequency of the oscillating circuit of the terminal.
In most cases, transponders have no autonomous power supply and extract the power supply necessary to their circuits from the high-frequency field radiated by the antenna of the terminal.
The quality of the communication and of the possible power transfer depends on the coupling between the terminal and the transponder. This coupling, which is inversely proportional (non linear) to the distance between the terminal and the transponder, conditions the amplitude of the voltage recovered by the transponder. It is thus needed to be able to evaluate the current coupling factor between a transponder and a terminal having it in its field.
The transponder thus recovers a power supply voltage which strongly depends on the coupling, and thus on the power consumption of the transponder circuits. For example, a decrease in the load (transponder power consumption) causes an increase of the voltage and of the remote-supply power. According to the current value of the coupling with respect to the optimum coupling, a load decrease may result in having the coupling approach the critical optimum coupling. The transponder then is in a paradoxical situation where its circuits consume less but where the voltage and the remote-supply power reach a maximum. For a current coupling k close to the optimum coupling, this maximum remote-supply power is not consumed by the transponder circuits and the power should thus be dissipated in the resonant circuit, thus causing an overheating of the antenna. This overheating may even cause the disconnection of the antenna contacts and thus the destruction of the transponder.
A conventional solution is to detune the transponder when the voltage across the oscillating circuit exceeds a threshold.
Causing a detuning risks worsening the situation by increasing the remote-supply power. Indeed, the detuning may be such that it corresponds to one of the maximum values of the transferred power for the detuned value. In this case, not only does the detuning not solve the problem, but it also worsens it. As a result, when modifying the transponder load by switching a resistor in parallel on the resonant circuit or when detuning said transponder by means of a switchable capacitance, without knowing the position of the current coupling with respect to the optimum coupling, there is a fifty-fifty chance to worsen the situation.